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MG MGF Technical - Pathetic engine power

I have recently fitted and had dynoed my new engine in my race car and the resuts have been both pathetic and disappointing to say the least.

The specification is as follows (in brief)

VVC Head with standard size SS valves and solid lifters.
Head ported (lightly) and cleaned up per DVA suggestions
Piper BP300M cams fitted to both inlet and exhaust (no VVC mechanism present)
Standard compression
Standard induction at this stage
Standard exhaust at this stage
Aftermarket ECU fully mapped and running on the throttle position (not MAP)

My intention was to map it in the format above to "prove" the mechanical integrity of the unit and then proceed with throttle bodies and extractors.

The results were as I said above pathetic. The max output was 67kw at the rear wheels at 6000rpm and diminishing rapidly thereafter.

My intial thoughts are that I have over-cammed it for the standard compression. We have poor fuel in Australia and I was cognizant of the need to be conservative in that regard.

Does anybody have any suggestions as to why it might be sucha poor output?
d mottram

For the Brits this is 89bhp. (KW to Bhp x by 1.3410)

What was the fuel air ratio?
Steve Ratledge

300's have a lot of duration, this could easily lead to next to no power and very high exhaust HC, especially without changeing the compression ratio. Did you fit verniers and time the cams?
Will Munns

David, I would seriously suspect the cam timing.

Is there any way of posting the curves somewhere? (Or send them to me, and I'll put them on the register website)
Rob Bell

Will, verniers were fitted and timed as per DVA suggestions at 130/110 LATDC. I will be checking the cam timing this coming weekend.
d mottram

That seems to be pretty immense lift..... On my highly ported DVA head with Piper 633 (10mm lift and 270deg duration (?) )the timing is set to 65/47 ATDC.

If you look at the Piper link of dave's site you will see this:

Cam RPM range, Timings Lift Lift mm Duration, VC LC Comments Lift@TDC
BP330M 3500-9500 44-66 0.460 11.68 290 .008" 101 Piston clearances 4.25mm

So it looks as though your timings are WAY out and close to piston/ valve contact :0

Tim
tim woolcott

Tim, the cams are B300M and as detailed below, Dave A suggests a LATDC around the figures I have used.

"Posted 25 July 2005 at 10:07:17 UK time
The physical limit of the plastic plenum is around 170-180BHP, frankly running BP300M cams on a plenum equipped engine is madness. The lift at TDC numbers for BP300M cams should be 130-140 thou inlet and 110-120 thou exhaust, these will equate to around 102 ATDC and 106-108 BTDC respectively. For these cams to work correctly you need unfettered induction (TBs or similar) and a good 4-2-1 mainfold. The duration of the BP300Ms is not so long as to compromise the effective compression stroke so you should see good results, a higher CR might yield a little more torque here and there though."

The quote is from the Elise Forum where I also posted my query. I have checked my piston/valve clearance when building the engine and it was quite acceptable.

I think that it would appear that my problem is the lack of TBs and a decent exhaust but I am still more than open to suggestions.

Regards..........David


d mottram

Gosh, I missed the fact that you were running a standard plenum and throttle body.

I think that you have your answer there (or at least in sizeable part) - you simply are not going to get any decent airflow. The cams and head that you're using is clearly optimised towards the top end of the rev range - precisely where your exhaust and and inlet manifolds are limiting. Effectively, you are getting the worst of both worlds: poor bottom end power due to big ports, high lift/duration cams causing the airflow to stall; poor top end power due to flow restriction from the standard bolt on parts.

If you have to keep the standard induction and exhaust, you may actually gain power by fitting milder cams - perhaps the 633s as Tim has on his?
Rob Bell

Just a thought..

If the standard plenum and exhaust will comfortably cope with the standard vvc output of 143bhp, and rev to 7k, why is it not doing so with this configuration?

The power drops off at 6k, which is still well within the working range of the standard exhaust and induction system, and at the point that the cams should be getting into their stride. I would expect the engine to underperform to its potential certainly, but not fail to match even standard spec.

It sounds more like an old fashioned ignition timing issue to me.. I'd look at your aftermarket ECU set up..

hope this helps

Mike
mike

It doesn't quite work like that Mike - although the point you make regarding mapping is a good one that I'd certainly go along with - but I am sure that David has done a good job there.

The manifold mismatch is all about air flows. Too big ports will lead to the airflow rate being very low; the air stream effectively stalls. This destroys low end torque.

At the top end, you need to promote good rate of airflow - but with restrictions, the head and cams never come on song... again destroying the torque and power at the top end.

I do suspect that David may be better off with milder cams, but failing that, a small investment in throttle bodies and a decent 4-2-1 exhaust manifold should reap significant rewards :o)
Rob Bell

This thread was discussed between 25/07/2005 and 26/07/2005

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